The need for coherent laser infrared radar (LIDAR) transmitters to maintain single-mode operation within a narrow frequency spectrum has stimulated the development of the present invention. Although LIDAR systems operating at wavelengths in the mid-IR region of the spectrum are to be preferred in many applications for eye safety, application of the present invention is by no means limited to lasers in the IR region, or for that matter to LIDAR systems.
Coherent LIDAR technology has been used for the profiling of aerosol and cloud backscatter, Doppler wind field measurements, and the measurement of atmospheric trace species. For example, during the NASA Global Backscatter Experiment Missions of 1989 and 1990, the Jet Propulsion Laboratory Airborne Backscatter LIDAR used a TEA-CO.sub.2 pulsed laser as its transmitter.
Coherent detection was used in order to achieve the sensitivity necessary to measure low backscatter coefficients at ranges of several kilometers, and a narrow bandwidth (10 MHz) electronics were used in the receiver chain to reduce total noise, thereby enhancing the signal-to-noise ratio and to suppress signal frequencies exceeding the sample rate parameters of the system digitizers.
Single-mode operation of the laser transmitter in the airborne backscatter LIDAR system is achieved by employing the technique of injection seeding. Depending on the injection intensity and transmitter pulsed laser cavity length, the injection zone of the pulsed laser can typically scan a frequency range of up to 15 MHz.
In order to maintain the pulsed laser frequency within the narrow bandwidth of the electronics, a transmitter stabilization loop is used. The transmitter stabilization loop uses a manually operated offset potentiometer to fine tune the pulsed laser frequency. The single-mode injection detuning zone is determined with respect to the offset potentiometer. During past operations, this potentiometer was manually adjusted to the midregion of the injection locking zone in order to insure that the transmitter frequency was within the receiver passband.
Although the stabilization loop keeps the burst frequency within the receiver passband, there was no way of insuring that the laser transmitter would be centered within this bandpass region on a burst-by-burst basis. Centering the pulse laser frequency in the receiver passband would optimize the signal-to-noise ratio and also avoid small but not insignificant signal variations which could effect the system calibration in a manner difficult to quantify.
Since the major objective of the GLOBE missions on a DC-8 research aircraft was quantitative measurement of aerosol and cloud backscatter, calibration and instrument stability were of utmost importance. Consequently, it is necessary to monitor the laser pulse frequency with respect to a local oscillator, i.e., to monitor the RF signal from a heterodyne receiver and make "fine tune" adjustments through the offset potentiometer in the stabilization loop to adjust the laser frequency in the mid-IR region (.lambda.&gt;1.5.mu.m) as necessary to maintain the RF signal within a narrow band on a pulse-by-pulse basis.
Frequency counters and spectrum analyzers that measure frequency are available, but they are unsuitable for this particular purpose. This is primarily because of the high laser pulse rate and short burst frequency variations of each laser burst which are too rapid for real-time observation. Therefore, an object of this invention is to provide a burst-by-burst frequency monitoring system and display system for real-time observation.